1. Technical Field
The subject invention relates to the identification and isolation of genes that encode enzymes (e.g., Thraustochytrium aureum Δ4-desaturase, Schizochytrium aggregatum Δ4-desaturase and Isochrysis galbana Δ4-desaturase) involved in the synthesis of polyunsaturated fatty acids and to uses thereof. In particular, Δ4-desaturase catalyzes the conversion of, for example, adrenic acid (22:4n-6) to ω6-docosapentaenoic acid (22:5n-6) and the conversion of ω3-docosapentaenoic acid (22:5n-3) to docosahexaenoic acid (22:6n-3). The converted products may then be utilized as substrates in the production of other polyunsaturated fatty acids (PUFAs). The product or other polyunsaturated fatty acids may be added to pharmaceutical compositions, nutritional composition, animal feeds as well as other products such as cosmetics.
2. Background Information
Desaturases are critical in the production of long-chain polyunsaturated fatty acids that have many important functions. For example, polyunsaturated fatty acids (PUFAs) are important components of the plasma membrane of a cell, where they are found in the form of phospholipids. They also serve as precursors to mammalian prostacyclins, eicosanoids, leukotrienes and prostaglandins.
Additionally, PUFAs are necessary for the proper development of the developing infant brain as well as for tissue formation and repair. In view of the biological significance of PUFAs, attempts are being made to produce them, as well as intermediates leading to their production, in an efficient manner.
A number of enzymes, most notably desaturase and elongases, are involved in PUFA biosynthesis (see FIG. 1). For example, elongase (elo) catalyzes the conversion of γ-linolenic acid (GLA) to dihomo-γ-linolenic acid (DGLA) and of stearidonic acid (18:4n-3) to (n-3)-eicosatetraenoic acid (20:4n-3). Linoleic acid (LA, 18:2n-9,12 or 18:2n-6) is produced from oleic acid (18:1-Δ9) by a Δ12-desaturase. GLA (18:3n-6,9,12) is produced from linoleic acid by a Δ6-desaturase.
It must be noted that animals cannot desaturate beyond the Δ9 position and therefore cannot convert oleic acid into linoleic acid. Likewise, γ-linolenic acid (ALA, 18:3n-9,12,15) cannot be synthesized by mammals. However, γ-linolenic acid can be converted to stearidonic acid (STA, 18:4n-6,9,12,15) by a Δ6-desaturase (see PCT publication WO 96/13591 and The FASEB Journal, Abstracts, Part I, Abstract 3093, page A532 (Experimental Biology 98, San Francisco, Calif., Apr. 18-22, 1998); see also U.S. Pat. No. 5,552,306), followed by elongation to (n-3)-eicosatetraenoic acid (20:4n-8,11,14,17) in mammals and algae. This polyunsaturated fatty acid (i.e., 20:4n-8,11,14,17) can then be converted to eicosapentaenoic acid (EPA, 20:5n-5,8,11,14,17) by a Δ5-desaturase. EPA can then, in turn, be converted to ω3-docosapentaenoic acid (22:5n-3) by an elongase. Isolation of an enzyme or its encoding gene, responsible for conversion of ω3-docosapentaenoic acid to docosahexaenoic acid (22:6n-3) has never been reported. Two pathways for this conversion have been proposed (see FIG. 1 and Sprecher, H., Curr. Opin. Clin. Nutr. Metab. Care, Vol. 2, p. 135-138, 1999). One of them involves a single enzyme, a Δ4-desaturase such as that of the present invention. In the n-6 pathway, dietary linoleic acid may be converted to adrenic acid through a series of desaturation and elongation steps in mammals (see FIG. 1). Production of ω6-docosapentaenoic acid from adrenic acid is postulated to be mediated by the Δ6-desaturase discussed above.
Other eukaryotes, including fungi and plants, have enzymes which desaturate at carbon 12 (see PCT publication WO 94/11516 and U.S. Pat. No. 5,443,974) and carbon 15 (see PCT publication WO 93/11245). The major polyunsaturated fatty acids of animals therefore are either derived from diet and/or from desaturation and elongation of linoleic acid or γ-linolenic acid. In view of these difficulties, it is of significant interest to isolate genes involved in PUFA synthesis from species that naturally produce these fatty acids and to express these genes in a microbial, plant, or animal system which can be altered to provide production of commercial quantities of one or more PUFAs.
In view of the above discussion, there is a definite need for the Δ4-desaturase enzyme, the respective genes encoding this enzyme, as well as recombinant methods of producing this enzyme. Additionally, a need exists for oils containing levels of PUFAs beyond those naturally present as well as those enriched in novel PUFAs. Such oils can only be made by isolation and expression of the Δ4-desaturase gene(s).
All U.S. patents and publications referred to herein are hereby incorporated in their entirety by reference.